1. Field of the Invention
The present invention, inter alia, relates to a low-voltage operation constant-voltage circuit. More specifically, it relates to a low-voltage operation constant-voltage circuit excellent in temperature characteristics and capable of being operated at a relatively low power supply voltage, such as, e.g., at about 1 V, and obtaining a stable output voltage, such as, e.g., about 0.6 V, regardless of the ambient temperature changes.
2. Description of the Related Art
The following description describes the knowledge of the inventor regarding the related art and the problems and therefore should not be interpreted that the inventor acknowledges them as prior art.
In the recent years, there exist many products configured to be operated at a relatively low-voltage to reduce the size and weight. In these types of products, a reference voltage which is low in voltage and stable in operation is required to operate the circuit in the product. As a circuit configured to obtain a stable output voltage, conventionally known is a band-gap reference voltage circuit configured to create a constant current source having a positive temperature coefficient and cancel the positive temperature coefficient of the voltage appeared at both ends of a resistance and the negative temperature coefficient of the base-emitter voltage of a diode-connected bipolar transistor (see, e.g., Japanese Patent Publication No. 2,734,964 and Japanese Patent Publication No. 2,745,610).
A conventionally known typical band-gap reference voltage circuit using bipolar transistors is shown in FIG. 12. This reference voltage circuit includes a first transistor Q1 having a unit emitter area, a second transistor Q2 having m-times emitter area (“m” is a positive number) and having an emitter resistance R1, a diode-connected third transistor Q3, a current mirror circuit including a diode-connected fourth transistor Q4 and a fifth transistor Q5 for self-biasing of the transistors Q1 and Q2, and a sixth transistor Q6 having a base to which the collector of the transistor Q5 is connected, wherein the first to third transistors are commonly connected with each other at their bases. This band-gap reference voltage circuit is configured such that the collector of the transistor Q6 drives the transistor Q3 through a resistor R2 to obtain an output voltage VREF.
The base-emitter voltage VBE3 of the third transistor Q3 has negative temperature characteristics, and the collector current I of the transistor Q3 has positive temperature characteristics, and therefore the temperature characteristics appeared at both ends of the resistor R2 become positive. Therefore, by connecting the resistor R2 and the third transistor Q3 in series, the positive temperature coefficient of the collector current I and the negative temperature coefficient of the base-emitter voltage VBE3 are cancelled, causing a stable output voltage regardless of the ambient temperature changes.
However, in the conventional reference voltage circuit, the reference voltage must be equal to the energy band-gap voltage (about 1.2 V) to make the temperature coefficient zero, and therefore only about 1.2 V output voltage can be extracted, and that the power supply voltage must be higher than that voltage (e.g., about 2 V). For this reason, the circuit cannot be operated at a low power supply voltage. Furthermore, the output voltage of the circuit is high, and therefore it could not be used as a reference voltage source for, e.g., a reset circuit for a microcomputer which requires a low reference voltage, such as, e.g., about 0.6 V.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.